A bit about oaths -

The Oath of Office is a solemn oath taken by officers of the United States Uniformed Services on commissioning.

It differs slightly from that of the oath of enlistment that enlisted members recite when they enter the service.
It is statutory ( required by law) and is prescribed by Section 3331, Title 5, United States Code.
It is traditional for officers to recite the oath upon promotion but as long as the officer's service is continuous this is not actually required. 

One notable difference between the officer and enlisted oaths is that the oath taken by officers does not include any provision to obey orders; while enlisted personnel are bound by the Uniform Code of Military Justice to obey lawful order, officers in the service of the United States are bound by this oath to disobey any order that violates the Constitution of the United States.

This the oath for commissioned officers:

"I, _____ (SSAN), having been appointed an officer in the Army of the United States, as indicated above in the grade of _____ do solemnly swear (or affirm) that I will support and defend the Constitution of the United States against all enemies, foreign or domestic, that I will bear true faith and allegiance to the same; that I take this obligation freely, without any mental reservations or purpose of evasion; and that I will well and faithfully discharge the duties of the office upon which I am about to enter; So help me God." (DA Form 71, 1 August 1959, for officers.)(Source)

Note that, "So help me God," is not required to be said if the speaker has a personal or moral objections, as is true of all oaths administered by the United States government. Also note that this is not an oath to defend any specific territory or persons or property. 

This is an oath to defend the Constitution of the United States. 
Note also that there is no duration defined in the Oath.

This is the oath for enlisted personnel:

"I, _____, do solemnly swear (or affirm) that I will support and defend the Constitution of the United States against all enemies, foreign and domestic; that I will bear true faith and allegiance to the same; and that I will obey the orders of the President of the United States and the orders of the officers appointed over me, according to regulations and the Uniform Code of Military Justice. So help me God." (Title 10, US Code; Act of 5 May 1960 replacing the wording first adopted in 1789, with amendment effective 5 October 1962)(Source) 

This the presidential oath of office:

"I do solemnly swear (or affirm) that I will faithfully execute the office of the President of the United States, and I will to the best of my ability, preserve, protect and defend the constitution of the United States."
(Source)